High-voltage power transformer is the most basic measurement equipment which provides measurement, control and relay protection for the power system. With the continuous improvement of the power system voltage levels, the traditional electromagnetic inductive or capacitive voltage divider transformer is difficult to overcome many limitations due to its restrictions of sensing mechanism. Optical voltage transformer is a novel transformer which performs voltage measurement using photonics technology and optical fiber sensing technology. Compared with the traditional transformers, in the optical voltage transformer high voltage signal is transmitted to the secondary equipment via optical fiber, so that insulation is greatly simplified, bandwidth is high, dynamic range is wide and it is non-magnetic saturation, lightweight and easy to install, therefore the optical voltage transformer has a very broad application prospects in the power system.
Internationally, companies such as Switzerland ABB, France Alstom, Canada Nxtphase, etc., have put optical voltage transformers to the market, and their principles of work are all using the longitudinal modulation structure based on Pockels electro-optic effect. In 1997, ABB Power T&D company reported 115 kV˜550 kV Combined Optical Voltage/current transformers, whose measurement accuracy reached IEC 0.2 level. In 1997, the French Alstom reported 123 kV˜765 kV Combined Optical Voltage/current transformers whose measurement accuracy can be up to 0.2% and multiple products have been linked to network to operate in Europe and North America. In 2003, Canada Nxtphase reported 121 kV 550 kV optical voltage transformer whose measurement accuracy can be up to IEC 0.2 level and IEEE 0.3 level. In China, some universities such as Tsinghua University, Huazhong University of Science and Technology, etc and many enterprises such as 26th Institute of China Electronics Technology Group Corporation, China Electric Power Research Institute, Shanghai Transformer Factory, etc have engaged in this research successively since 1992, and now many types of optical voltage transformer prototype have been developed but the overwhelming majority of them are limited to laboratory phase.
Currently, most of the existing optical voltage transformers have the problems of variation with temperature of the light source light-emitting wavelength and instability of output power; the existing optical voltage transformers also have the problems of large size, heavy weight, high cost, the phenomenon of partial discharge and withstanding voltage, and the additional insulation design is required, therefore the hidden dangers due to pressure, humidity and other factors are increased, and the long-term safety and reliability can not be ensured well; in the bonding process, the multiple discrete optical components will inevitably produce micro outward movement during the curing process, which affecting the accuracy of azimuth; and the follow-up electronic circuits will directly bring error to the optical signal due to the drift caused by temperature changes, which reduces the stability of the system; in addition, as far as modulation scheme is concerned, compared with horizontal modulation, longitudinal modulation optical voltage transformers also have the problems of high cost of insulation under high pressure, out of proportion between the output signal and the electric field and difficult signal demodulation.